This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. We have an ongoing program to use crosslinking to probe for proteins that interact with a target protein both in vitro and in vivo and ultimately to determine the sites of interaction between these proteins. We have written software that will help us to deduce this information by mass spectrometric analysis of the crosslinked proteins after digestion by various proteases. Currently we are testing the use of formaldehyde and glutaraldehyde for this purpose.